Complex sounds may be localized and, in the case of human speech, understood in the presence of background noise and across wide variations in level. The resilience of perceptual experience exceeds the coding capacity of individual auditory nerve fibers (ANFs) but is well correlated with representations that are observed in the ventral cochlear nucleus (VCN). Because the auditory nerve is the sole conduit of neural information from the ear to the brainstem, these coding enhancements must be achieved by integrating responses across many ANFs. The long-term goal ofthis application is to explore the "selective listening" model of ANF/VCN convergence, which proposes a hypothetical mechanism for the context-dependent optimization of ANF inputs. The model assumes that sensitive high spontaneous rate ANFs dictate spectral representations at low sound levels because less sensitive low spontaneous rate ANFs are limited by threshold effects. Conversely, low spontaneous rate fibers dominate high sound levels because saturated inputs from high spontaneous rate fibers are switched off by shunting inhibition. Although selective listening is a conceptual model, two recently described populations of VCN multipolar neurons offer a putative anatomical substrate for exploring the physiological reality of the hypothesized network. Planar neurons (coding neurons) are richly innervated by ANFs and display enhanced spectral representations. Radiate neurons (switching neurons) immunolabel for glycine and display connectivity that is capable of altering the efficacy of ANF inputs to planar neurons. Electrophysiological recordings of Aim 1 will use parameterized stimuli to quantify the spectral coding properties of planar neurons before and after pharmacological manipulations reversibly block glycinergic inhibition. Tract-tracing experiments of Aim 2 will isolate radiate neurons as a primary source of planar inhibition and support their role in shunting inhibition by defining the topography of their inputs relative to the dendritic fields of planar neurons. PUBLIC HEALTH RELEVANCE: Central processing disorders negatively impact hearing, language comprehension, and sound localization, particularly in complex acoustic environments. These deficits resist treatment with conventional hearing aids. A better understanding of adaptive sound processing in the VCN is likely to contribute to effective rehabilitation strategies.